Relative number densities of atomic and ion species with respect to the total density of baryons, as a function of redshift. The computation goes from z = 10⁴ to z = 10. Hydrogen species are represented as full lines, helium as dashed lines, and deuterium as dash-dotted lines. We indicate the recombination redshifts of primordial helium, deuterium and hydrogen nuclei (He²⁺, He⁺, then D⁺ and H⁺) with vertical dotted line.